It has been studied to apply a nitride semiconductor to a high withstand voltage and high-power semiconductor device by using characteristics such as high saturation electron velocity and a wide band gap. For example, the band gap of GaN being the nitride semiconductor is 3.4 eV, and it is larger than the band gap of Si (1.1 eV) and the band gap of GaAs (1.4 eV), and has a high breakdown electric field intensity. Therefore, GaN is extremely expectable as a material of a semiconductor device for power supply use in high voltage operation and obtaining high-power.
As a semiconductor device using the nitride semiconductor, there are a lot of reports as for a field-effect transistor such as a high electron mobility transistor (HEMT). For example, as a GaN based HEMT (GaN-HEMT), an AlGaN/GaN.HEMT using GaN as an electron transit layer, and AlGaN as an electron supply layer has been focused. In the AlGaN/GaN-HEMT, distortion occurs at AlGaN due to a lattice constant difference between GaN and AlGaN. High concentration two-dimensional electron gas (2DEG) is obtained by a piezoelectric polarization and a spontaneous polarization of AlGaN which occur due to the distortion. Accordingly, it could be used as a high withstand electronic device such as a high efficiency switch element and an electric vehicle.
Related-art techniques are disclosed in Japanese Laid-open Patent Publication No. 2013-12735, Japanese Laid-open Patent Publication No. 2008-34438 and Japanese Laid-open Patent Publication No. 2007-73656.
However, in the GaN-HEMT, a current collapse phenomenon where current decrease due to voltage stress is often observed, and lowering of output has been seen as a problem. The current collapse phenomenon occurs because electrons which are accelerated by a strong electric field in a vicinity of a gate are captured by electron traps existing in a semiconductor crystal, at a semiconductor surface, and in a surface passivation film.